Electronic systems such as Personal Computers (PCs) use dynamic-random-access memory (DRAM) memory chips mounted on small, removable memory modules. The original single-inline memory modules (SIMMs) have been replaced with dual-inline memory modules (DIMMs), and 184-pin RIMMs (Rambus inline memory modules) and 184-pin DDR (double data rate) DIMMs.
The memory-module industry is very cost sensitive. Testing costs are significant, especially for higher-density modules. Specialized, high-speed electronic test equipment is expensive, and the greater number of memory cells on high-speed memory modules increases the time spent on the tester, increasing costs.
Handlers for integrated circuits (ICs) have been used for many years in the semiconductor industry. Handlers accept a stack of IC chips that are fed, one at a time, to the tester. The tested IC is then sorted into a “bin” for IC chips that have passed or failed the test. More recently, handlers have been made for memory modules.
FIG. 1 shows a memory module handler connected to a high-speed electronic tester. Memory modules 18 to be tested are loaded into the top of handler 10 in the input stack. Memory modules 18 drop down, one-by-one, into testing area. Module-under test MUT 20 is next to be tested. Arm 26 pushes MUT 20 laterally until it makes contact with contactor pins 16 that clamp down on “leadless” connector pads formed on the substrate of MUT 20.
Contactor pins 16 are also connected to test head 14, which makes connection to tester 12. Tester 12 executes parametric and functional test programs that determine when MUT 20 falls within specified A.C. and D.C. parameters, and whether all memory bit locations can have both a zero and a one written and read back.
Rather than test memory modules, handler 10 can feed packaged integrated circuit chips to tester 12. For example, DRAM chips are typically tested using a handler and a tester before being assembled into memory modules.
Tester 12 can cost from ten-thousand to millions of dollars. Cost can be reduced if a less-expensive tester replaces tester 12. Since most memory modules are intended for installation on PCs, some manufacturers test memory modules simply by plugging them into memory module sockets on PC motherboards. A test program is then executed on the PC, testing the inserted module. Since PCs cost only about a thousand dollars, tester 12 and handler 10 of FIG. 1 are replaced by a low-cost PC. Equipment costs are thus reduced by a factor of a hundred.
FIG. 2 shows a PC motherboard being used to manually test memory modules. Substrate 30 is a motherboard. Components 42, 44, mounted on the top side of substrate 30, include ICs such as a microprocessor, logic chips, buffers, and peripheral controllers. Sockets for expansion cards 46 are also mounted onto the top or component side of substrate 30.
Memory modules 36 are SIMM, DIMM, or other kinds of memory modules that fit into SIMM/DIMM sockets 38. SIMM/DIMM sockets 38 (hereinafter SIMM sockets 38) have metal pins that fit through holes in substrate 30. These pins are soldered to solder-side 34 of substrate 30 to rigidly attach SIMM sockets to the PC motherboard. Both electrical connection and mechanical support are provided by SIMM sockets 38.
While using PC motherboards for testing memory modules greatly reduces equipment costs, labor costs are increased. Memory modules must be inserted and removed manually. Manual insertion and removal of memory modules is slow and labor-intensive.
U.S. Pat. No. 6,178,526 teaches that the component side of the PC motherboard is too crowded for attaching a SIMM/DIMM handler. The inventors realized that the back or solder-side of the PC motherboard is less crowded and provides unobstructed access. The PC motherboard is modified to provide reverse attachment of the memory-module handler to the solder-side of the PC motherboard using a handler adapter board. The SIMM socket on the component side of the PC motherboard is removed, and the handler adapter board is plugged from the backside into the holes on the PC motherboard for the SIMM socket.
Thus memory modules can be automatically inserted and removed from the adapter board that is reverse-mounted. Many memory modules are being tested in this manner today using low-cost PC motherboard testers.
While using a PC motherboard to test memory modules is feasible, memory chips that have not yet been soldered onto a memory module cannot be tested using a PC motherboard, since there are no sockets for individual DRAM memory chips. The PC motherboard only has sockets for memory modules.
Newer memory modules are more complex than earlier memory modules. For example, a common memory module known as a fully-buffered dual-inline memory module (FB-DIMM) uses a serial interface that sends packets to and from the motherboard. An Advanced Memory Buffer (AMB) on the memory module deserializes the packets and generates local control signals to the DRAM chips. Thus the interface on the motherboard is a serial interface, rather than a DRAM-control-signal interface.
DRAM chips are typically tested individually using a chip handler and an expensive IC tester. The large number of DRAM chips tested in this manner helps to keep test costs low. However, still lower test costs are desirable. Testing individual DRAM chips on a low-cost PC motherboard tester could reduce costs compared with using an expensive automated-test-equipment (ATE) integrated circuit tester.
What is desired is a PC motherboard tester that can test individual DRAM chips before they are assembled onto a memory module substrate. A system to interface individual DRAM chips to a PC motherboard that uses memory modules is desirable. Testing individual memory chips on a standard PC motherboard that has a serial interface to support fully-buffered memory modules is also desirable.